1. Field of the Invention
The present invention relates to semiconductor fabrication. More particularly, the invention relates to a method for inspecting a reticle for use in photolithography, a method for making a reticle for use in photolithography, and a reticle for use in photolithography.
2. Description of the Related Art
As is well known to those skilled in semiconductor fabrication, photolithography involves selectively exposing regions of a resist-coated silicon wafer to a radiation pattern, and developing the exposed resist to either protect or expose regions of underlying wafer layers (e.g., regions of substrate, polysilicon, or dielectric). In the fabrication of semiconductor chips, one of the problems experienced during photolithography is that the features defined in the wafer layer are subject to corner rounding. One source of this problem is the rounding of features that occurs during the manufacturing of the reticle, which includes the photomask in which a pattern corresponding to features at one layer of an integrated circuit (IC) design is defined.
FIG. 1A shows a conventional stepper apparatus 10 used in photolithography. Stepper apparatus 10 includes radiation source 12, reticle 14, and focusing lens 16. As shown in FIG. 1A, stepper apparatus 10 is disposed above resist-coated wafer 18, which is divided into a plurality of dies 20. In operation, radiation 22 (e.g., light) from radiation source 12 is projected onto the surface of resist-coated wafer 18. After passing through reticle 14 and focusing lens 16, radiation 22 contacts the surface of resist-coated wafer 18 within one of dies 20 and defines IC design pattern 24xe2x80x2 in the die surface. This process is typically repeated until an IC design pattern has been defined in each of dies 20 by stepping stepper apparatus 10 in two dimensions above resist-coated wafer 18. Thereafter, conventional development, etching, and stripping operations are typically performed to define the features at one layer of the IC design.
FIG. 1B shows a detailed view of conventional reticle 14 shown in FIG. 1A. As shown in FIG. 1B, reticle 14 includes transparent glass plate 26 on which photomask 28 is formed. Photomask 28, which is formed of chromium or other suitable light-blocking material, has IC design pattern 24 defined therein. IC design pattern 24 corresponds to features at one layer in an IC design. When radiation 22 from radiation source 12 is directed toward reticle 14, radiation (e.g., light) passes through pattern 24, which corresponds to the portion of transparent glass plate 26 not covered by photomask 28, and projects onto resist-coated silicon wafer 18 (see FIG. 1A) disposed below the reticle.
Reticles are presently manufactured by depositing a chromium photomask on a transparent glass plate, coating the photomask with a resist, defining a pattern in the resist using a pattern generator, developing the resist, and subjecting the photomask to chemical processing to remove everything but the desired pattern from the glass plate. In the operation in which the pattern is defined in the resist, the pattern generator directs an electron beam to define the desired features in the resist.
FIG. 2A shows an exemplary ideal IC design pattern 24 that may be defined in a photomask. The ideal pattern 24 has well-defined outside corners 30 and inside corners 32. The corners 30 and 32 form sharp, 90 degree angles. Unfortunately, when the pattern is defined in the photomask, these corners are subject to rounding due to electromagnetic wave effects and chemical processing effects.
FIG. 2B illustrates the corner rounding that may occur during preparation of a photomask in which ideal IC design pattern 24 shown in FIG. 2A is to be defined. As shown in FIG. 2B, actual IC design pattern 24xe2x80x2 has rounded outside corners 30a and rounded inside corners 32a. These corners do not form sharp, 90 degree angles and, consequently, the actual IC design pattern 24xe2x80x2 does not have the desired shape of ideal IC design pattern 24. This is undesirable because the discrepancy in shape of desired features, e.g., metallization features, may render numerous devices on a wafer inoperable and thereby decrease the manufacturing yield.
The deleterious effects of corner rounding become more problematic at smaller feature sizes. Thus, as the feature sizes in modem IC designs continue to decrease, it becomes increasingly more important to monitor closely all potential sources of corner rounding to avoid excessive yield losses. As noted above, one source of corner rounding is the rounding of corners that occurs in the process of manufacturing the reticle.
To date, several approaches have been used to inspect reticles for corner rounding. In one approach, an optical inspection device is used to compare the actual features of the photomask to corresponding data on a data tape used to prepare the photomask. This approach is undesirable because it requires complex equipment that is not only expensive, but also may be difficult and time consuming to install and operate. In another approach, an operator inspects the reticles under a microscope to determine visually whether the degree of corner rounding of features defined in the photomask is acceptable. This approach suffers from the disadvantage that it may be inconsistent or unreliable because it relies upon the operator""s subjective judgment to determine the acceptability of the reticle.
In view of the foregoing, there is a need for an inexpensive, consistent, and reliable method of inspecting reticles to determine whether the degree of corner rounding is within acceptable limits.
Broadly speaking, the present invention fills these needs by providing a photomask having a test pattern and a reference marker defined therein. When a reticle having the photomask formed thereon is inspected under a microscope, the crosshair of the microscope may be used in conjunction with the reference marker to evaluate the degree of corner rounding of a feature of the test pattern.
In one aspect of the invention, a method for inspecting a reticle to evaluate the degree of corner rounding of a feature of a test pattern is provided. In this method a reticle having a photomask formed thereon is placed under a microscope. The photomask has a pattern corresponding to features of a semiconductor chip design defined therein. In addition, the photomask further has a test pattern and a crosshair orientation mark defined therein. The test pattern has at least one test corner for evaluating a degree of corner rounding when the test pattern is defined in the photomask. The crosshair orientation mark is defined in the photomask to orient a crosshair of the microscope relative to the test pattern. Once the crosshair of the microscope is aligned with the crosshair orientation mark, the crosshair of the microscope is used to evaluate the degree of rounding of the test corner of the test pattern.
In another aspect of the invention, a method for inspecting a reticle to determine the pass/fail status of the reticle is provided. In this method, a reticle having a photomask formed thereon is placed under a microscope. The photomask has a pattern corresponding to features of a semiconductor chip design defined therein. The photomask further has a test pattern and a crosshair alignment mark defined therein. The test pattern has at least one test corner for determining a pass/fail status of the reticle. The crosshair alignment mark is defined in the photomask to orient a crosshair of the microscope relative to the test pattern. Once the crosshair of a microscope is aligned with the crosshair alignment mark, the crosshair of the microscope is used to determine the pass/fail status of the reticle.
In yet another aspect of the invention, a method for making a reticle is provided. In this method, a glass substrate is first provided. A photomask having a pattern corresponding to features of a semiconductor chip design is then generated. A cell that includes a test pattern and a reference marker is defined in the photomask. The reference marker is positioned relative to the test-pattern so that the reference marker can be used in conjunction with a crosshair of a microscope to inspect a feature of the test pattern. Finally, the photomask is formed on the glass substrate to provide a reticle.
In a still further aspect of the invention, a reticle for use in photolithography is provided. The reticle includes a glass substrate and a photomask formed on the glass substrate. The photomask has features of a semiconductor chip design, a test pattern, and a reference marker defined therein. The reference marker is positioned relative to the test pattern so that the reference marker can be used in conjunction with a crosshair of a microscope to inspect a feature of the test pattern.
The present invention advantageously provides a simple and inexpensive method for inspecting the features of a photomask formed on a reticle. The inspection methods of the present invention are reliable because they rely upon an objective standard for evaluating corner rounding of photomask features formed on a reticle. In addition, the present invention enables the uniformity of features formed at different locations on the reticle to be verified quickly and reliably.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.